Polyurethane dispersion (PUD) adhesives have been developed in recent years as an alternative to solvent-based adhesives. Typically the dispersion is prepared by dispersing an isocyanate functional prepolymer in aqueous media, and chain extending the prepolymer after it is dispersed. Compatibility is conventionally provided by means of ionic groups such as carboxylate or sulfonate groups, or both, provided on the backbone of the polyurethane polymer. Addition of crosslinkers, such as isocyanates, aziridines, melamine resins, epoxies, oxazolines or carbodiimides at the time of use to improve bond strength is known and effective. However, substitution of polyurethane dispersion adhesives for solvent-based adhesives in specific applications often presents a potential user with special problems.
References describing polyurethane dispersion adhesives include U.S. Pat. No. 4,870,129; U.S. Pat. No. 5,334,690; U.S. Pat. No. 5,432,228; U.S. Pat. 5,494,960; U.S. Pat. No. 5,523,344; U.S. Pat. No. 5,532,058; U.S. Pat. No. 5,608,232; U.S. Pat. No. 5,610,232; U.S. application Ser. No. 08/561197 filed Nov. 21 1995, allowed (equivalent to WO97/19121); U.S. application Ser. No. 08/961,752, filed Oct. 31 1997; and DE 3930352, all incorporated herein by reference.
PUD adhesives offer several advantages over solvent based polyurethane adhesives such as low VOC, less odor, less hazard of fire, explosion or acute toxic reaction, greater acceptability from an environmental perspective and ease of handling. On the other hand PUD adhesives have drawbacks, such as lower water resistance, chemical resistance and heat resistance. To date the primary approach to overcoming these drawbacks is to use a polyisocyanate crosslinker.
In bonding rubbery materials to themselves, or to other substrates commonly found in footwear, it is conventional practice to apply a primer of a chlorinating compound, typically trichloroisocyanuric acid to the rubbery substrate(s). Application of the primer has been found to consistently provide bonds which fail by a substrate failure mode, rather than an adhesive failure mode. Use of this type of primer is conventional with solvent adhesives and provides similar benefits for polyurethane dispersion adhesives aged in limited humidity environments.
In bonding footwear rubber materials it has been found that isocyanate crosslinked PUDs made will give excellent T-peel bond strengths when aged in limited humidity (50%) environments, but that strength significantly deteriorates upon ageing in high humidity and high temperature environments. This problem is found with PUDs which contain carboxylate groups, sulfonate groups or mixtures of both as aqueous compatibilizing agents.